What was found
FIELD ENGINEER REPORTED A SAFETY HAZARD OF THE PARKING BRAKE VALVE ASSY. IT IS THAT WHEN THE PILOT TOES THE BRAKE PEDALS NORMALLY THE PARKING BRAKE VALVE WILL CLOSE ITSELF. AND THE SAME MALFUNCTION HAS OCCURRED MANY TIMES IN THE PAST YEARS. AFTER DISASSEMBLING AND INSPECTING THE VALVE PARTS, FOUND THIS MALFUNCTION IS CAUSED BY THAT THE DECREASED FRICTION OF THE CAM AND O-RINGS CAN'T MEET THE SPRING FORCE OF THE 2 CHECK VALVES AND THIS IS BECAUSE OF THE LONG USE TIME OF THIS PARKING BRAKE VALVE. BUT DO NOT KNOW WHEN THIS MALFUNCTION WOULD OCCUR.